The invention relates generally to the field of honeycomb assemblies. In particular, the invention pertains to honeycomb assemblies including corrugated ribbons having stiffening ribs, their method of manufacture, and apparatus for their manufacture.
Honeycomb material has come into increasing use in fields of application where both strength and light weight are important. Honeycomb assemblies commonly include a sandwich of two face sheets between which a cellular web is fixed to form a unitary structure. Typically, the web is fixed to the face sheets by welding, brazing, glueing or some other similar process. The cellular web is composed of hexagonal or otherwise shaped cells formed by joining similarly configured corrugated ribbons. The ribbons are laterally displaced with respect to one another to form webs, as with natural honeycomb. Various materials are used for both the face sheets and the corrugated ribbons depending upon the application, including steel, aluminum, stainless steel, titanium, papers, graphite composites, and various alloys, both common and exotic.
U.S. Pat. No. 4,632,862, describes a dramatically improved honeycomb structure. That patent pertains to an I-beam honeycomb structure in which conventional honeycomb, having corrugated ribbons arranged to define, for example, six sided cells, is further formed by centrally depressing the major flat surfaces of the corrugated ribbons to form parallel sub-flat surfaces. The sub-flat surfaces are connected to the major flat surfaces by side walls which are perpendicular to both the major flats and the sub flats. Joining two or more such corrugated ribbons with alternate ribbons inverted, by bonding sub-flats to sub-flats and major flats to major flats, produces a honeycomb web which incorporates I-beams extending end-to-end through the web. The I-beams provide a tremendous degree of strength not found in conventional honeycomb.
One difficulty encountered with the fabrication of honeycomb assemblies generally, and I-beam honeycomb assemblies in particular, stems from the need to consistently form the corrugated ribbons which form the assembly. This requires precisely controlling the lengths of the flat sections of the ribbons as well as the angles between adjacent flats. In the case of I-beam honeycomb, these tasks are complicated by the smaller angles to which the ribbon must be bent, which in turn results in smaller cells being formed in greater numbers.
A host of variables in the starting ribbon contributes to problems in consistently corrugating the ribbon. For example, the ability to form metal ribbon is affected by the ribbon's thickness, width, temper, yield strength, strain hardening factor, and metallurgical structure. Additionally, variables in the forming process and equipment affect corrugation as well. These variables include the inner and outer bend radii applied to the ribbon, punch or die clearances, the coefficients of friction between the tool and the ribbon, and the coining force applied to the ribbon. Tool wear, and loading rate affect the forming process as well.
Ribbons formed of titanium alloys have proven particularly difficult to corrugate because of their high yield strengths, strain hardening rates, and springback ratios. Springback occurs due to residual stresses in the regions of the ribbons which are bent during corrugation. These stresses are present in part because some of the material in the bent regions is deformed elastically while some of the material is deformed plastically. The material which is deformed elastically tries to "straighten out" the bends, while the material which is deformed plastically, and therefore strain-hardened, struggles to retain the bends. Consequently, the angles of corrugated ribbons formed of titanium alloys typically do not match those of the tools used to form them.
To reduce springback, forming tools should have an inner bend radius which is less than the thickness of the starting ribbon material. The edges of the tools, however, wear away quickly, particularly when forming ribbons made of high strength material such as stainless steel, inconel, and titanium. While springback can also be reduced by building forming equipment specific for each combination of cell shape, cell size, ribbon material, ribbon thickness, and ribbon width, this is not economically feasible. For example, 280 sets of tooling would be required to make separate tooling to form each iteration of honeycomb ribbon, in 0.001" increments, from 0.002" to 0.015 thick", in each of four different cell shapes, and in each of five different cell sizes.
Accordingly, it is an object of the invention to provide a honeycomb assembly having a more consistent cell shape than known honeycomb structures. Another object of the invention is to provide a method of manufacturing such a structure. Still another object of the invention is to provide an apparatus for manufacturing such a structure.